It is well known in the prior art pertinent to this invention to provide double-containment systems having inner piping or carrier components contained within outer piping or containment components in order to transport, for example, dangerous or hazardous fluids within the inner piping or carrier components. In the event of a leak or emission of fluid or vapors from the inner piping, the leaking substance may be safely contained within the outer piping or containment components. These types of systems have found widespread use in the nuclear, gas and petroleum refining, and chemical processing industries.
In most double-containment systems, there are a plurality of gaps formed between adjacent sections of outer piping and/or other containment components, which are provided to access joints in the inner piping and/or carrier components in order to form the joints and to later leak test, pressure test or otherwise test the integrity of the carrier components and/or their joints. For example, it is considered good piping practice when installing and testing pipes for chemical use to provide such gaps in order to perform hydrostatic pressure tests on the primary pipe joints and to allow visual access to these joints when performing such tests. Accordingly, once the joints for the inner piping or carrier components are completed, such as by welding, solvent-cement bonding or heat fusion, and the primary system tests are completed, there are corresponding gaps formed between the outer piping or containment components.
It is a common problem or difficulty to close these gaps in an efficient and effective manner when assembling double-containment systems. The known methods or fittings for closing gaps formed between the ends of double-containment piping assemblies are referred to as "mid-line closures", and include: (i) split-pipe sections, also referred to as "clam-shell" couplings, (ii) weld wraps, (iii) sheet wraps, and (iv) slip couplings. Examples of each of these types of mid-line closures are illustrated in Christopher G. Ziu, Handbook of Double Containment Piping Systems .sctn. 11.1.2.4 (1995).
Although these types of closures have been problematic for double-containment piping systems made with many types of materials, they have been particularly problematic in thermoplastic applications. For polypropylene (PP) and high density polyethylene (HDPE) applications, the most common methods for joining the circumferential ends in each of these types of mid-line closures, and for joining the longitudinal seams in the weld wrap and clam-shell closures, have been hot-gas welding, and to a lesser extent, extrusion welding. Polyvinylidene fluoride (PVDF) has also been a widely used material in double-containment piping systems, and is likewise a heat-fusible thermoplastic that may be joined in this manner. For PVC applications, solvent cementing and adhesive bonding have also been used to join the seams in these types of closures, and in fiberglass applications, adhesive bonding and butt-and-wrap techniques have been used.
One of the problems with these types of joining methods, is that they frequently produce a weld (or bond) quality factor of less than one (meaning that the joints are substantially weaker in some cases than the base material itself). For example, in the case of hot-gas welding, a commonly used method for joining PP and HDPE materials, a weld quality factor of approximately 0.1 to 0.4 may be expected on a long-term basis, meaning that the joint will have only about 10% to 40% of the strength of the base material.
One of the most efficient methods of joining thermoplastic piping components made of PP and HDPE is electrical-resistance welding (also referred to as "electro-fusion") wherein an electrical coil is interposed between the edges of the components forming the seam, and an electric current is passed through the coil to heat and thereby fuse the components together along the seam. Although electrical-resistance welding has been used to join the circumferential seams of polypropylene slip couplings in double-containment systems, this has produced inconsistent results with less than desirable repeatability. With slip couplings, it is difficult to properly pre-position the electric coils at the locations of the circumferential seams, and to in turn slip the coupling in place over the coils such that the parts are precisely located to thereby form a high-integrity seam. Because of practical difficulties and relatively imprecise tolerances, this approach has lead to inconsistent and unreliable results. Accordingly, although electrical-resistance welding or electro-fusion has been used to a limited extent to join circumferential seams in polypropylene slip couplings, the present inventor is not aware of these methods having been proposed or successfully applied to join the seams of clam-shall couplings or for to otherwise join the horizontal or longitudinal seams in double-containment closure fitting assemblies.
Another problem encountered in double-containment piping systems is differential thermal expansion and contraction, which occurs when the inner and outer piping components expand or contract to different degrees, or at different rates relative to each other. Almost all double-containment systems are subject to changes in temperature during operation, and/or to differences in operating temperature between the inner and outer piping components. This in turn causes differential thermal expansion and contraction which can subject the piping components to bending and/or torsional movements, intensified stress, and in some cases failure.
In order to accommodate such differential thermal expansion and contraction, flexibility supports have been developed for supporting the inner-piping within the outer piping and for permitting axial, lateral and/or radial movement of the inner piping relative to the outer piping to prevent the accumulation of undesirable stress in the piping components. Typical flexibility supports are illustrated in U.S. Pat. No. 5,197,518 to Christopher G. Ziu, entitled "Centering Support Assembly For Double Containment Pipe Systems", and in U.S. Pat. No. 5,482,088 to Christopher G. Ziu, entitled "Double-Containment Systems With Axial-Guiding and Flexibility Supports".
One of the difficulties encountered in installing flexibility supports, is that they are typically placed mid-line in the double-containment piping assemblies, thus requiring gaps to be cut or otherwise formed in the outer piping or containment components for installation of the flexibility supports, and in turn requiring closure fittings to close the gaps. This can be a relatively time consuming and difficult procedure.
It is an object of the present invention to overcome the drawbacks and disadvantages of the prior art double-containment piping systems, and of the closure fitting and flexibility support assemblies of such systems.